Focal plane arrays (FPAs) are used in various applications to capture images for subsequent processing. For example, non-line of sight launch system (NLOS-LS) precision attack missiles (PAMs) have employed uncooled infrared (UCIR) sensors with a focal plane array to capture infrared images for use in target tracking. The focal plane array has a matrix of infrared detector elements arranged in a matrix of rows and columns yielding an n row by m column focal plane array. An exemplary UCIR sensor may have a focal plane array of 640×480 detectors. The output of each detector may be referred to as a pixel.
Each detector may have a slightly different sensitivity to infrared radiation than other detectors. For example, the output of some pixels may be too bright or too dark for a given amount of incident radiation. This non-uniform sensitivity yields fixed pattern noise (FPN). Fixed pattern noise may manifest itself in the output image of the sensor by resulting in a non-uniform response across the image. Thus, fixed pattern noise leads to degradation in target recognition, acquisition and tracking.
Some attempts to compensate for fixed pattern noise have been made using a rudimentary approach to non-uniformity compensation (NUC). This approach uses a simplistic algorithm that adjusts each pixel with an offset value. The offset values are generated by determining an amount of change needed to place each pixel at a mid-gray level in response to a given input. In practice, the offset values are calculated in response to a uniform input image on the focal plane array (e.g., a “bland” and “smeared” image generated by de-focusing imaging optics). Once the offset values are generated, the system images in a normal manner while applying the offset values to the outputs of each pixel. While simple, this approach causes unneeded biases in overall image level and potentially creates artifacts (e.g., an after-image burn-in of the input scene that was used during the creation of the offset values). Also, this technique does not address pixels that are defective.